Sendmail: Relay Messages to Ext. SMTP Server?

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http://www.linuxquestions.org – I've got a server set up with Debian. The problem is that my ISP doesn't allow usage of port 25 (as I understand a lot of ISP's don't). I have PHP scripts hat use the mail() function to send e-mails through the Sendmail application. So, my question is: How can I relay messages sent to sendmail to an external SMTP server (like Google Mail, for example)? Would this be easier to do in the PHP configuration, or on the Sendmail side of things? I've used PEAR extensions in PHP to achieve this before, but scripts downloaded from the Internet generally just use the basic mail() function, and it would be a real pain to modify them all. (HowTos)